Home » Learning » Java » Lesson 5: Java 8 Date & Time API (java.time Package)
JavaLearning

Lesson 5: Java 8 Date & Time API (java.time Package)

javasleep javasleep
March 13, 2025
23 Views

Before Java 8, date and time handling in Java was done using java.util.Date and java.util.Calendar, which had several problems:
❌ Mutable objects – Modifying a Date object would change the original instance.
❌ Thread safety issues – SimpleDateFormat was not thread-safe.
❌ Confusing APIs – Working with time zones and date calculations was complex.

Java 8 introduced the java.time package, which provides a more robust, immutable, and easy-to-use Date & Time API.

1. Key Classes in java.time Package

ClassDescriptionExample
LocalDateRepresents a date (year, month, day)LocalDate.now() → 2025-02-10
LocalTimeRepresents a time (hours, minutes, seconds)LocalTime.now() → 14:30:15.123
LocalDateTimeRepresents date and timeLocalDateTime.now() → 2025-02-10T14:30:15.123
ZonedDateTimeRepresents date-time with a time zoneZonedDateTime.now(ZoneId.of("Asia/Tokyo"))
InstantRepresents a timestamp (time since Unix epoch)Instant.now() → 2025-02-10T05:30:15.123Z
DurationRepresents an amount of time (e.g., “2 hours”)Duration.ofHours(2)
PeriodRepresents a date-based amount of time (e.g., “3 years, 2 months”)Period.ofYears(3)
DateTimeFormatterFormats date and time objects into stringsDateTimeFormatter.ofPattern("yyyy-MM-dd")

2. Getting the Current Date & Time

Let’s see how we can retrieve the current date and time using Java 8:

📌 Example: Get the Current Date

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println("Current Date: " + today);
    }
}

✅ Output:

Current Date: 2025-02-10

✔ LocalDate.now() returns the current date without time.

📌 Example: Get the Current Time

import java.time.LocalTime;

public class Main {
    public static void main(String[] args) {
        LocalTime now = LocalTime.now();
        System.out.println("Current Time: " + now);
    }
}

✅ Output:

Current Time: 14:35:27.456

✔ LocalTime.now() returns only the time, including hours, minutes, seconds, and nanoseconds.

📌 Example: Get Both Date & Time (LocalDateTime)

import java.time.LocalDateTime;

public class Main {
    public static void main(String[] args) {
        LocalDateTime dateTime = LocalDateTime.now();
        System.out.println("Current Date & Time: " + dateTime);
    }
}

✅ Output:

Current Date & Time: 2025-02-10T14:35:27.456

✔ LocalDateTime.now() combines date and time but does not include a time zone.

3. Formatting Dates (DateTimeFormatter)

By default, Java prints date-time in ISO 8601 format (yyyy-MM-ddTHH:mm:ss). To customize it, use DateTimeFormatter.

📌 Example: Custom Date Formatting

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class Main {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
        String formattedDate = today.format(formatter);
        
        System.out.println("Formatted Date: " + formattedDate);
    }
}

✅ Output:

Formatted Date: 10-02-2025

✔ DateTimeFormatter.ofPattern("dd-MM-yyyy") customizes the date format.

4. Parsing Strings to Dates

We can convert a string to a date using DateTimeFormatter.

📌 Example: Convert a String to a LocalDate

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class Main {
    public static void main(String[] args) {
        String dateString = "15-08-2025";
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
        LocalDate date = LocalDate.parse(dateString, formatter);
        
        System.out.println("Parsed Date: " + date);
    }
}

✅ Output:

Parsed Date: 2025-08-15

✔ The string "15-08-2025" is converted into a LocalDate object.

5. Adding & Subtracting Time

MethodDescriptionExample
plusDays(n)Adds n daysdate.plusDays(5)
minusMonths(n)Subtracts n monthsdate.minusMonths(2)
plusHours(n)Adds n hourstime.plusHours(3)

📌 Example: Add and Subtract Dates

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        LocalDate nextWeek = today.plusDays(7);
        LocalDate lastMonth = today.minusMonths(1);

        System.out.println("Today: " + today);
        System.out.println("Next Week: " + nextWeek);
        System.out.println("Last Month: " + lastMonth);
    }
}

✅ Output:

Today: 2025-02-10
Next Week: 2025-02-17
Last Month: 2025-01-10

✔ We used plusDays() and minusMonths() to modify dates.

6. Working with Time Zones (ZonedDateTime)

LocalDateTime does not store time zones, but ZonedDateTime does.

📌 Example: Get Current Time in a Specific Time Zone

import java.time.ZonedDateTime;
import java.time.ZoneId;

public class Main {
    public static void main(String[] args) {
        ZonedDateTime tokyoTime = ZonedDateTime.now(ZoneId.of("Asia/Tokyo"));
        System.out.println("Current Time in Tokyo: " + tokyoTime);
    }
}

✅ Output:

Current Time in Tokyo: 2025-02-10T23:45:30.123+09:00[Asia/Tokyo]

✔ ZoneId.of("Asia/Tokyo") gives us the current time in Japan.

Lesson Reflection

  1. Why is java.time better than java.util.Date and Calendar?
  2. How does immutability improve the reliability of date/time objects?
  3. Can you think of a real-world scenario where ZonedDateTime would be useful?

Example: Convert UTC Date-Time to Central Time (CT – America/Chicago)

The ZonedDateTime class allows us to convert between time zones easily. Central Time (CT) corresponds to the America/Chicago time zone in Java.

📌 Convert UTC Date-Time to Central Time (CT – America/Chicago)

import java.time.ZonedDateTime;
import java.time.ZoneId;
import java.time.LocalDateTime;

public class Main {
    public static void main(String[] args) {
        // Step 1: Create a LocalDateTime (without timezone)
        LocalDateTime localDateTime = LocalDateTime.of(2025, 2, 10, 14, 30); // 2:30 PM
        
        // Step 2: Convert LocalDateTime to UTC (Coordinated Universal Time)
        ZonedDateTime utcDateTime = localDateTime.atZone(ZoneId.of("UTC"));
        
        // Step 3: Convert UTC to Central Time (CT - America/Chicago)
        ZonedDateTime ctDateTime = utcDateTime.withZoneSameInstant(ZoneId.of("America/Chicago"));
        
        // Output results
        System.out.println("UTC Time: " + utcDateTime);
        System.out.println("Central Time (CT): " + ctDateTime);
    }
}

✅ Example Output (Central Time during Standard Time)

UTC Time: 2025-02-10T14:30Z[UTC]
Central Time (CT): 2025-02-10T08:30-06:00[America/Chicago]

✔ 2025-02-10T14:30Z (UTC) converts to 2025-02-10T08:30-06:00 (CT -6:00).

🔄 Explanation

  1. Create a LocalDateTime → 2025-02-10T14:30 (No timezone).
  2. Assign UTC Time Zone → ZonedDateTime.of(localDateTime, ZoneId.of("UTC")).
  3. Convert to CT (America/Chicago) → withZoneSameInstant(ZoneId.of("America/Chicago")).
  4. Print Results → Shows UTC and CT converted time.

⏳ What Happens During Daylight Saving Time (DST)?

Central Time (CT) follows Daylight Saving Time (CDT, UTC-5) from March to November. If the input date falls in this period, the offset changes from -06:00 to -05:00 automatically.

✅ Example Output (During DST – July 10, 2025)

UTC Time: 2025-07-10T14:30Z[UTC]
Central Time (CT): 2025-07-10T09:30-05:00[America/Chicago]

✔ Java automatically adjusts for daylight saving time (DST).

Key Takeaways

✔ ZonedDateTime allows timezone conversion easily.
✔ Use withZoneSameInstant() to adjust time zones correctly.
✔ Handles Daylight Saving Time (DST) automatically.

the next Java 8 feature: Optional Class (java.util.Optional)

Tags:

Related Articles

Java Sleep

Links

About Us

Java Sleep